Atrial synchronized, dual chamber, pacing modes, particularly, the multi-programmable, VDD, VDDR, DDD and DDDR pacing modes, have been widely adopted in implantable dual chamber pacemakers for providing atrial and ventricular or AV synchronized pacing on demand. Such dual chamber pacing modes have also been incorporated into implantable cardioverter/defibrillators (ICDs) and into right and left heart pacing systems providing synchronized right and left heart pacing for enhancing left ventricular cardiac output as described in commonly assigned U.S. Pat. No. 5,902,324.
Such pacing systems are embodied in an implantable pulse generator (IPG) adapted to be subcutaneously implanted and at least atrial and ventricular pacing or cardioversion/defibrillation leads that are coupled to the IPG. The atrial and ventricular leads each incorporate one or more lead conductor that extends through the lead body to an exposed pace/sense electrode or cardioversion/defibrillation electrode disposed in operative relation to a heart chamber.
The pacing operating system comprises atrial and ventricular sense amplifiers and atrial and ventricular pace pulse generators. The atrial sense amplifier is coupled to active and indifferent atrial pace/sense electrodes to detect electrical signals of the heart associated with atrial depolarizations (P-waves) and to generate an atrial sense event (A-EVENT) signal when detection criteria are met. The ventricular sense amplifier is coupled to active and indifferent ventricular pace/sense electrodes to detect electrical signals of the heart associated with ventricular depolarizations (R-waves) and to generate a ventricular sense event (V-EVENT) signal when detection criteria are met. The atrial pace pulse generator delivers a negative-going or cathodal voltage atrial pacing (A-PACE) pulse through a pacing path comprising the active and indifferent atrial pace/sense electrode. The ventricular pace pulse generator delivers a negative-going or cathodal voltage atrial pacing (V-PACE) pulse through a pacing path comprising the active and indifferent ventricular pace/sense electrode.
The pacing operating system times out various intervals from each A-EVENT, V-EVENT, A-PACE, and V-PACE to maintain synchronous depolarizations of the atria and ventricles. Such AV synchronous pacemakers that perform this function have the capability of tracking the patient's natural sinus rhythm and preserving the hemodynamic contribution of the atrial contraction over a wide range of heart rates. Maintenance of AV mechanical synchrony is of great importance as set forth in greater detail in commonly assigned U.S. Pat. No. 5,626,623.
Each of the A-PACE and V-PACE pulse energies is set at a programmable energy level, involving both pulse width (duration) and amplitude (strength), so as to provide sufficient energy to cause the heart chamber to depolarize and contract. The minimum pacing pulse energy which is required to capture and thus evoke a muscular depolarization within the heart is referred to as the “stimulation threshold”, and generally varies in accordance with the well known strength-duration curves, wherein the amplitude of a stimulation threshold current pulse and its duration are inversely proportional. When a delivered pacing pulse is successful in so stimulating the heart into contraction, it is said to have “captured” the heart, whereas failure to stimulate the heart is described as “loss of capture” (LOC).
In order to maximize the useful life of dual chamber pacing IPGs, it is desirable that the A-PACE pulse energy and the V-PACE pulse energy be programmed to the minimal energies required to capture the atria and ventricles, respectively. As is well known, the stimulation threshold for a patient, in both the atrium and the ventricle, can fluctuate both short term and long term following implantation. The A-PACE and V-PACE pulse energies are therefore typically programmed by the physician at implantation employing an external programmer to exceed the stimulation threshold by a “safety margin” to avoid atrial loss of capture (ALOC) and ventricular loss of capture (VLOC).
As described in commonly assigned U.S. Pat. No. 5,324,310, the post-operative determination of the stimulation thresholds by the physician typically requires the patient to be connected to surface ECG equipment while a threshold routine is conducted using the pacemaker programmer. The pacemaker programmer successively reprograms the pulse width and/or amplitude on a temporary basis to ascertain the points at which capture is lost. The A-PACE and/or V-PACE pulses are observed on a display or paper tracing as spikes, and capture or LOC is observed by the presence or absence of the “evoked response” wave shape (a P-wave or an R-wave) that follows each spike. At LOC, the programmed pacing pulse may be immediately restored so that the patient does not experience syncope. A strength-duration curve may be plotted from the resulting threshold data. The resulting threshold data may then be used to permanently reprogram the pulse energy. Naturally, such periodic patient studies are time-consuming and expensive to conduct. Moreover, they do not provide an indication of stimulation threshold fluctuation over the course of a patient's day and levels of activity.
Therefore, systems and methods have been proposed to be incorporated or have been incorporated into the IPG operating system to periodically, automatically conduct stimulation threshold tests, and to readjust the pacing pulse energy in relation to any newly determined stimulation threshold. See, for example, U.S. Pat. No. 3,920,024, where the pacing pulse energy is initially set at a high enough energy to ensure capture, and then is reduced by successive increments until LOC is detected and a back-up pacing pulse is delivered. The evoked response characteristic of capture is intended to be sensed across a particular pace/sense electrode pair, and LOC is inferred if the evoked response is not detected within a short time window following delivery of the pacing pulse.
However, sensing of the evoked response is rendered difficult for a variety of reasons. The A-PACE and V-PACE pulses are produced by the exponential discharge of respective atrial and ventricular output capacitors through the impedance loads in the atrial and ventricular pacing paths that each include a coupling capacitor, the active and indifferent pace/sense electrodes, and the patient's heart tissue between the pace/sense electrodes. It is conventional to suppress or blank both of the atrial and ventricular sense amplifiers during A-PACE and V-PACE pulses for blanking periods to avoid overloading the sense amplifier, to allow a fast recharge function to be completed, and to prevent sensing of artifacts resulting in false declarations of A-EVENTs or V-EVENTs.
In addition, a number of sense amplifier refractory periods are timed out on atrial and ventricular sense event signals and generation of A-PACE and V-PACE pulses, whereby “refractory” A-EVENT and V-EVENTs during such refractory periods are selectively ignored or employed in a variety of ways to reset or extend time periods being timed out. The durations of the blanking and refractory periods therefore render it difficult to reliably detect an evoked response, if any, across the same pace/sense electrode pair that the A-PACE or V-PACE pulse is delivered across.
As a result of these considerations, a great deal of effort has been expended over many years to develop IPGs having the capability of automatically testing the stimulation threshold, i.e. providing an “auto-capture” detection function, and resetting the pacing pulse energy to exceed the threshold by the safety margin without the need for clinical or patient intervention.
Commonly assigned U.S. Pat. Nos. 6,134,473 and 6,144,881 describe the Capture Management algorithm implemented, for example, in the Medtronic® Kappa® 700 pacemaker IPGs. The polarity of the positive or negative change in voltage with respect to time (or dv/dt) of the waveform incident on the pace/sense electrodes is monitored during a short period of time immediately following a paced event. In one embodiment, sensing of the evoked response is based upon a relationship between a maximum magnitude of a derivative of a sensed signal and a predetermined threshold reference value. The evoked response is declared when the maximum amplitude of the derivative of the sensed signal equals or exceeds the threshold reference value.
The Capture Management algorithm is periodically run, e.g., once a day at a prescribed time to perform a pacing threshold search (PTS) wherein the pacing pulse amplitudes and pulse widths of the pacing pulses delivered in each pacing channel are incrementally adjusted within a predetermined range to determine the pacing threshold, and the threshold data is stored for analysis of long term trends. When an “Adaptive” mode of the Capture Management algorithm is programmed the Capture Management algorithm automatically adjusts the pacing pulse amplitude and/or pulse width setting to ensure capture at minimum pacing energy while maintaining the programmed safety margin(s).
A wide variety of other approaches have been taken as reflected by the extensive listing of earlier patents described in the above-referenced '310 patent and in commonly assigned U.S. Pat. Nos. 5,320,643, 5,324,310, 5,331,966, 5,601,615, 5,683,431, 5,861,012, 5,861,013, and 6,231,607, for example, and in further U.S. Pat. Nos. 4,686,988, 5,165,404, 5,165,405, 5,172,690, 5,222,493, 5,285,780, 5,564,430, and 5,683,426, for example.
The '310 patent, for example, discloses employing additional capture detection sense amplifiers and sense electrode pairs to detect the evoked response within an anticipated time following delivery of an A-PACE or V-PACE pulse. In other approaches, as exemplified by the above referenced '643 patent, one or more physiologic sensors that show a response to the mechanical action of the heart, e.g. a piezoelectric or impedance sensor, or that show changes in physical properties of the blood when the heart is captured, e.g. blood pH, temperature, impedance or blood pressure sensors on the pacing lead have also been suggested. In virtually all of these approaches, it is necessary to rely on additional components and circuitry that consume more energy and add to the bulk and cost of the system and raise reliability issues.
In one atrial auto-capture approach disclosed in U.S. Pat. No. 5,476,486, for example, the amplitudes of a series of A-PACE pulses are progressively decremented, and the presence or absence of a V-EVENT within an AV delay is noted. The absence of a V-EVENT indicates ALOC.
In one embodiment of the above-referenced '615 and '012 patents particularly for use with patients having intact and regular A-V conduction and with or without an intrinsic atrial sinus rhythm, A-PACE pulses are delivered at a test escape interval (that is shorter than any intrinsic atrial interval) and a paced AV (PAV) delay is timed out. An ALOC is declared in the absence of a detected V-EVENT in the latter portion of the test PAV delay following the delivery of the A-PACE test pulse. In the ventricular threshold test regimen, a V-PACE test pulse is delivered after a shortened test PAV delay timed from the preceding A-PACE pulse. A VLOC is declared by the detection of a V-EVENT in the ventricular refractory period of the delivered V-PACE pulse.
These atrial capture detection methods depend upon normal AV conduction, which is not present in many instances where the patient is pacemaker dependent in the ventricles. Therefore, these methods cannot be used if the patient has a high degree of AV block since an A-PACE triggered atrial depolarization is not conducted reliably to the ventricles to trigger a ventricular depolarization sensed as a V-EVENT.
In a second embodiment of the '615 and '012 patents, for use in the atrium or ventricle in patients having intrinsic sinus rhythm, each test A-PACE or V-PACE pulse is delivered at a test escape interval set as a fraction, e.g., about 50%–75% of the average intrinsic escape interval timed from a preceding A-EVENT or V-EVENT, respectively. A sense test window set to be somewhat longer than the test escape interval is timed out from delivery of the test A-PACE or V-PACE pulse. ALOC or VLOC is declared if an A-EVENT or V-EVENT, respectively, is detected within the sense test window. A-CAPTURE or V-CAPTURE is declared if an A-EVENT or V-EVENT, respectively, is detected after time-out of the sense test window. The energy of the test A-PACE pulse is successively incremented or decremented until an A-EVENT is detected within the sense test window, which signifies ALOC at the test energy of the previously delivered A-PACE test pulse. Preferably, in this embodiment, the test energy in pulse width and amplitude of the A-PACE or V-PACE test pulses is increased until A-CAPTURE or V-CAPTURE is declared so that the patient's normal rhythm is not disturbed frequently during the test.
These atrial and ventricular auto-capture methods depend upon the presence of normal sinus rhythm in the atrium and ventricles, respectively, which is not present in many instances where the patient is pacemaker dependent in one or both of the atria and the ventricles. In such a case, the sense test window cannot be determined since intrinsic A-EVENTs and V-EVENTs cannot be reliably detected.
Further auto-capture methods disclosed in U.S. Pat. Nos. 5,476,487, 5,674,254, and 6,216,037, rely upon the observation that the QT interval of the PQRST waveform varies in duration as a function of capture or LOC of the ventricles following a V-PACE or the atria following an A-PACE conducted to the ventricles. The commonly assigned '037 patent discloses an implantable DDD pacemaker incorporating a method for continually determining whether a delivered A-PACE pulse has resulted in capture of the atrium. A V-PACE is delivered after the time-out of a PAV delay, and the following QT interval is determined. An atrial depolarization that spontaneously occurs during the time-out of the PAV can affect the QT interval. The QT interval is measured on a cycle-by-cycle basis, stored, and employed in making a determination of change in QT interval, i.e., )QT=*QT−QTPREV*. The )QT variable is compared to a lower limit, e.g., 2 ms, and a higher limit, e.g., 10 ms, and when )QT is within this range there is a determination of ALOC.
Detecting the QT interval following delivery of a V-PACE using the ventricular sense amplifier is itself difficult given the ventricular blanking and refractory intervals. Moreover, this approach presumes that the atria have an intrinsic depolarization rate that is overdriven at a faster atrial pacing rate. It also assumes that there is intact AV conduction, but that the ventricles are also paced. Typically, atrial and ventricular synchronized pacing is necessary during AV block and when the intrinsic atrial heart rate is lower than is desirable to provide sufficient cardiac output to meet the patient's needs.
What is needed in the art of determining ALOC is a capability of reliably detecting when a delivered A-PACE pulse has evoked an atrial depolarization without requiring special tests or additional components or circuitry that consume energy and add to the bulk and cost of the system. Specifically, there is a substantial need in a pacing system for an atrial capture detection method and system that function simply and reliably when the patient is pacemaker dependent in the atria or in both the atria and the ventricles.